Device and process for heating a primary material

ABSTRACT

In a device ( 1 ) for heating primary materials, in particular calcined petroleum cokes ( 5, 6 ) from which an anode ( 4 ) can be manufactured for dry electrolysis, comprising a mixing and forming unit ( 8 ) in which the primary materials ( 5, 6 ) can be formed into anodes ( 4 ) which are subsequently fired in an annular anode furnace ( 3 ), the thermal waste heat of the annular anode furnace ( 3 ) should be usable for heating up the primary materials ( 5, 6 ).  
     This is achieved in that the primary materials ( 5, 6 ) can be heated in a heat exchanger ( 11 ) by means of flue gas drawn from the annular anode furnace ( 3 ) through a feed pipe ( 12 ), and that the petroleum coke fractions ( 5, 6 ) heated in this way can be input into the mixing and forming unit ( 8 ) for forming the unfired anode ( 4 ).

The present invention relates to a device, a control system and a process for heating a primary material, in particular calcined petroleum coke, in accordance with the pre-characterising clauses of patent claims 1, 11 and 14.

To date, anodes for dry electrolysis used in the manufacture of primary aluminium in accordance with the Hall-Héroult process have been produced in such a way that the primary material, in particular calcined petroleum coke, is supplied to a mixing unit or kneader. Furthermore, a binding agent is added to the mixing unit. The binding agent usually consists of mineral coal tar pitch and is processed together with the calcined petroleum coke to produce a paste that is formed into a “green anode” and then fired in an annular anode furnace.

In the annular anode furnace, the anodes undergo three phases, namely a heating-up phase, a firing phase and a cooling-down phase. This is achieved by means of the movement of the extraction, heating and cooling devices in a specified translocation cycle above the anodes that are arranged stationary in pits and are sealed off from the ambient air.

The preheating of the petroleum coke improves its wettability and reduces the porosity of the unfired anode. The petroleum coke is heated by means of a heat carrier oil that is supplied to the mixing unit. This oil is heated to the required temperature in an oil or gas-fired heating unit. This heating process requires considerable energy.

To date, the waste heat contained in the flue gas from the annular anode furnace has been supplied to a flue gas scrubber where it is cleaned and cooled. This means the energy of the waste gas is largely unused and given off to the environment.

It is therefore the task of the present invention to provide a device of the aforementioned type, by means of which the waste heat created in the annular anode furnace can be used for heating the primary material used for manufacturing the anode. Furthermore, a control system should be provided by means of which the utilisation of the waste heat from the annular anode furnace can be thermally optimised. In addition, the present invention is intended to specify a process by means of which the heat is provided to the unfired anode. The flue gas cleaning of the annular anode furnace should also be improved.

These tasks are accomplished in the present invention by the features of the pre-characterising clauses of patent claims 1, 11 and 14.

Other advantageous further embodiments of the present invention are described in the subordinate claims.

Hot flue gas is drawn from the heating-up zone of the annular anode furnace and, possibly, waste heat from its cooling-down zone and is transferred by means of a heat exchanger to the petroleum coke primary material that is to be heated up, therefore the thermal energy previously existing in the annular anode furnace is used for manufacturing the unfired anodes. This results in a significant energy saving, because the energy that is required to heat up the petroleum coke before it is supplied to the mixing unit in order to improve the wettability of the petroleum coke does not have to be provided by an additional heating unit.

Part of the waste heat generated in the cooling-down zone of the annular anode furnace can be supplied via extraction units 31 into the feed pipe through which the flue gas is drawn into the heat exchanger, therefore the cooling of the anodes in the cooling-down zone is improved because the heated air in the cooling down zone is transported out more rapidly. Furthermore, the temperature in the furnace hall is lower because the extracted waste heat from the cooling-down zone no longer heats up the immediate vicinity. This improves the working conditions in the area of the annular anode furnace.

The drawing shows a schematic sample embodiment configured in accordance with the present invention, the details of which are explained below. The drawing specifically shows:

A device and a control system for utilizing the waste heat from an annular anode furnace that is transferred to a primary material in a heat exchanger.

The waste heat of an annular anode furnace 3 is to be utilised in a thermally self-contained device 1 that is monitored by a control system 2 and controlled in accordance with a process control schematic described in more detail below. In the annular anode furnace 3, green anodes 4 are initially heated to a particular temperature in a part of the furnace 3 that functions as the heating-up zone 22. In the firing zone 23 of the annular anode furnace 3 provided subsequent to this, the anodes 4 placed in the furnace are exposed to an elevated temperature for a certain specified period of time. In a cooling-down zone 24, the anodes 4 are cooled so that they can subsequently be used in an electrolysis cell for manufacturing primary aluminium.

The temperature in the firing zone 23 is generated by means of burners 25 that are schematically illustrated. The three zones 22, 23 and 24 are thermally interconnected so that the air heated by the hot anodes flows out of the cooling-down zone 24 into the firing zone 23 and finally into the heating-up zone 22 where it heats up the anodes 4 placed in that zone.

The normal primary material used for forming the anode 4 is calcined petroleum coke 5 or 6 that is mixed together in a defined way from various grain fractions. The calcined petroleum coke 5, 6 is mixed with a binding agent 7 made from a mineral coal tar pitch 7 and supplied to a mixing and forming unit 8 in which the petroleum coke fractions 5 and 6 as well as the binding agent 7 are processed into a paste and formed into the unfired anode 4.

The device 1 is intended to make it possible to use the waste heat arising in the annular anode furnace 3 for heating up the petroleum coke 5 and 6. Furthermore, the device 1 can incorporate a system component by means of which the pre-cleaned and cooled flue gas from the annular anode furnace 3 is passed through a flue gas cleaning unit.

The flue gases drawn from the heating-up zone 22 are channelled through a feed pipe 12 to a heat exchanger 11. The calcined petroleum coke 5 or 6 is placed in the heat exchanger 11 and the flue gas flows directly through or around it. The heat exchanger 11 can have individual units for the various petroleum coke fractions 5 or 6, into which units it is possible to place the respective petroleum coke fractions 5 or 6. The heat exchanger 11 in this case is configured as a fluidized bed or multiple bed heat exchanger so that the input petroleum coke fractions 5 or 6 pass through the heat exchanger 11 at a certain speed and are heated up by the flue gas flowing in as they do so. The petroleum coke 5 or 6 heated in this way is next supplied to the mixing and forming unit 8 together with the binding agent 7 that is loaded into the mixing and forming unit 8 following preliminary heating by other means.

The temperature differential between the heated petroleum coke 5 and 6 and the temperature of the flue gas 13 coming from the heat exchanger is at least 50 K. The control system 2 is electrically connected to a temperature sensor 19 by means of which the temperature of the flue gas drawn from the heating-up zone 22 is measured. If the control system 2 detects that the temperature of the flue gas in the feed pipe 12 is too low, waste heat can be input from the cooling-down zone 24 of the annular anode furnace 3 via the gas regulator 29 of an extraction unit 31 that is connected to the cooling-down zone 24 of the annular anode furnace 3. Negative pressure exists in the feed pipe 12, therefore there is no need for fans. Furthermore, the extraction unit 31 is connected to the ambient air via another gas regulator 30, so that ambient air can be admixed with the waste heat drawn from the cooling-down zone 24 if necessary, in order to reduce it to a certain specified temperature.

In addition, the flue gas in the feed pipe 12 can be further heated by a heater 20 that is also switched on or off by the control system 2. A gas regulator 27 is provided directly ahead of the heat exchanger 11 and this gas regulator 27 makes it possible to control the inlet cross section of the feed pipe 12 into the heat exchanger 11. As a result, the quantity of flue gas flowing into the heat exchanger 11 can be controlled by the gas regulator 27. If only a partial flow of the flue gas is required for heating, the remainder is channelled by a gas regulator 29 integrated in bypass pipe 21 with the effect that this flue gas flows directly out of the feed pipe 12 around the heat exchanger 11. Furthermore, the heat exchanger 11 is provided with an output pipe 13 through which the waste gases are passed to the flue gas cleaning unit 14. The bypass pipe 21 emerges in the output pipe 13 with the effect that all flue gases and the waste heat from the annular anode furnace 3 are channelled through the feed pipe 12 and the heat exchanger 11 into the output pipe 13 and therefore into the flue gas cleaning unit 14 or alternatively around the heat exchanger 11 through the bypass pipe 21 into the output pipe 13. However, the bypass pipe 21 can also emerge in a separate part of the flue gas cleaning unit 14 via a separate pipe 9, in order to achieve a smaller pressure loss for the partial flow in the bypass pipe 21.

It has proven to be particularly advantageous for the temperature of the anode 4 formed by the mixing and forming unit 8 to be about 160° C. This temperature of the anode 4 is measured by a temperature sensor 19 and the temperature value of the anode 4 after forming is transmitted to the control system 2. Further temperature sensors 19 measure the temperature of the petroleum coke 5 and 6 leaving the heat exchanger 11 and transmit the temperature values measured in this way to the control system 2. The temperature values obtained for the petroleum coke 5 and 6 as well as for the anode 4 after forming enable the control system 2 to calculate what temperature is required in the feed pipe 12 and to set the regulators 29, 30 of the extraction unit 31 accordingly.

For operation of the annular anode furnace 3, the burners 25 are changed over according to a specified cycle so that the former firing zone 23 becomes the cooling-down zone 24.

After the burners 25 have been changed over, the temperature of the flue gas channelled into the heat exchanger 11 through the feed pipe 12 is at its lowest but the temperature in the cooling-down zone 24 is at its highest, therefore the contrarotating temperature sequence itself achieves a certain extent of equilibrium because the flue gas mixes with the waste heat drawn from the cooling-down zone 24. The temperature required in the feed pipe 12 is achieved precisely by the control system 2 and the position of the gas regulators 29 and, if necessary, 30. Also, the maximum temperatures prevailing in the feed pipe 12 can be restricted in this way.

In the event of a malfunction in the annular anode furnace 3, the petroleum coke 5 and 6 is heated up by the heater 20 integrated in the feed pipe 12.

The flue gas undergoes physical and chemical sorption processes as the petroleum coke 5 or 6 flows through the heat exchanger 11, therefore the heat exchanger itself performs a pre-separation function as part of the flue gas cleaning. The fluidised bed reactor 15 installed in the flue gas cleaning unit 14, which can be operated with special sorption substances such as ureas NH3 for NOx separation and/or activated charcoal for PAH separation depending on the requirements in terms of purified gases and can be configured as a multiple bed reactor if necessary, means that the flue gas is completely purified. The fluidised bed reactor 16 operated with limestone chippings eliminates sulphur and fluorine residues from the flue gas. In accordance with the present invention, it is also possible to use a sorption reactor instead of the fluidised bed reactor 15 with the ultrafine fraction of the petroleum coke 5 or 6 as the sorption agent.

It is advantageous for the pitch vapours 10 from the mixing and forming unit 8 to be added to the flue gas in the output pipe. This obviates the need for a separate cleaning unit which would otherwise be required.

The sample embodiment is based on an energy cycle as specified below by way of example. The relationship between the anode mass flow and the flue gas mass flow in the annular anode furnace 3 is between 1:3 and 1:7, on average 1:5, as a result of the proportion of secondary air in the flue gas. The specific heat values of the petroleum coke 5 and 6 are approximately the same, which means that the flue gas mass flow that is many times that of the petroleum coke mass flow contains sufficient energy to heat up the petroleum coke 5 and 6. The binding agent becomes liquid at a temperature of about 200° C. and is admixed with the primary material, i.e. the petroleum coke fractions 5 and 6, before they enter the heat exchanger, thereby supplying a part of the necessary heating energy. This means it is sufficient for the petroleum coke 5 and 6 to have an initial temperature of about 150° C. after the heat exchanger 11.

The temperature of the flue gas in the feed pipe 12 is influenced by the number of burners moving around the anode furnace 3 and fluctuates between about 150° C. and about 250° C. during the course of a burner transfer cycle. The temperature differential between the flue gas and the petroleum coke 5 and 6 at the output of the heat exchanger 11 should be about 50 K, therefore it is necessary to ensure that the temperature of the flue gas flowing into the heat exchanger 11 is about 200° C. Given these assumptions and a mass flow of petroleum coke 5, 6 of 12.5 t/h with a low green anode wastage rate, it is necessary to have a flue gas mass flow through the heat exchanger 11 of about 30 t/h. The flue gas mass flow in the feed pipe 12 is about 70 t/h with an average proportion of secondary air, the unrequired proportion amounting to about 40 t/h is channelled around the heat exchanger 11 via the bypass pipe 21.

With a 3-burner annular anode furnace 3, a cycle time of 24 h and with changeovers of the burners 25 performed at evenly spaced intervals, the flue gas temperature profile will have a saw-toothed shape with a periodicity of 8 hours. This involves the flue gas temperature in the feed pipe 12 rising from about 150° C. to about 250° C. and then falling back to 150° C. when the burners 25 are next changed over.

In order to guarantee that the flue gas achieves a minimum temperature of 200° C. before it enters the heat exchanger 11, the process in accordance with the present invention involves hot air at about 400° C. being extracted intermittently from the cooling-down zone 24 of the annular anode furnace 3 by extraction units 31 and being admixed with the flue gas in the feed pipe 12. In the aforementioned example, this amounts to about 14 t/h immediately after the changeover of the burners 25 followed by a linear decreasing tendency until zero after 4 hours.

A triple-cascade closed-loop control system based on a control unit is provided in order to ensure effective process control. Initially, the main control parameter is obtained by measuring the temperature of the green anode 4, which means when the anode 4 leaves the mixing and forming unit 8. Depending on this value, the gas regulator 28 integrated in the bypass pipe 21 and the gas regulator 27 located in the feed pipe 12 directly ahead of the heat exchanger 11 are set by the control system 2 so that the changing flue gas volumetric flows in the heat exchanger 11 result in the required petroleum coke starting temperature being achieved. In an advantageous embodiment, this is achieved by a closed cascade control loop using the coke temperature as a secondary control parameter. The additional volume of hot air drawn from the cooling-down zone 24 by the extraction units 31 means that the entire mass flow of the flue gas is controlled using a gas regulator 30 connected to the atmosphere in such a way that the additional air drawn from the cooling-down zone is minimised, i.e. hot air from the cooling-down zone 24 can only be admixed if the petroleum coke temperature can no longer be achieved by the temperature of the flue gas drawn exclusively from the heating-up zone 22.

The thermal processes take place relatively slowly, therefore the control system 2 is based on an intelligent control unit that sets the actuator values of the individual gas regulators 27 to 30 in advance based on data collected empirically for specified operating statuses, and the control system 2 only has to make slight adjustments to take account of the actual status of the device 1. The control process in accordance with the present invention is preferably based on a neural net or a neuro-fuzzy algorithm. 

1. A device (1) for heating calcined petroleum coke (5, 6) from which an anode (4) can be manufactured for dry electrolysis, the device comprising a mixing and forming unit (8) in which calcined petroleum coke (5, 6) is formed into anodes (4) for subsequent firing in an annular anode furnace (3), wherein the device further comprises a heat exchanger (11) for heating the calcined petroleum coke (5, 6), and a feed pipe (12) by means of which flue gas is taken from the annular anode furnace (3), and the petroleum coke fractions (5, 6) heated in this manner can be put into said mixing and forming unit (8) for forming into the unfired anode (4).
 2. The device in accordance with claim 1, wherein said heat exchanger (11) comprises a fluidized or moving bed heat exchanger and/or that said heat exchanger (11) comprises at least one unit in which petroleum coke (5, 6) of different grain fractions can be directly exposed to the heated flue gas from the annular anode furnace (3).
 3. The device in accordance with claim 1, wherein an extraction unit (31) with a controllable gas regulator (29, 30) is attached to said feed pipe (12) through which the flue gas is channelled into said heat exchanger (11), by means of which the hot air from a cooling-down zone (24) is channelled into said feed pipe (12).
 4. The device in accordance with claim 1, wherein a bypass pipe (21) is integrated in said feed pipe (12) ahead of said heat exchanger (11) and through which the flue gas is guided around said heat exchanger (11).
 5. The device in accordance with claim 3, wherein said feed pipe (12) is connected to a selected one of a heating-up zone (22) and the cooling-down zone (24) of the annular anode furnace (3).
 6. The device in accordance with claim 5, wherein the controllable gas regulator (30) is integrated in the extraction unit (31) connected to the cooling-down zone (24) of the annular anode furnace (3) and ambient air flows in through the gas regulator (30), and the waste heat from the cooling-down zone (24) is input into said feed pipe (12) in a controllable fashion by means of the gas regulator (29).
 7. The device in accordance with claim 1, wherein an auxiliary heater (20) for generating hot air is integrated into said feed pipe (12), by means of which the flue gas in said feed pipe (12) is heated to a selected temperature level before the flue gas enters said heat exchanger (11).
 8. The device in accordance with claim 1, wherein said heat exchanger (11) is connected to an output pipe (13) that emerges in a cleaning unit (14) configured as a selected one of a multiple bed reactor (15), a fluidized bed reactor (16), and a dust separator (17) comprising a cyclone or textile filter.
 9. The device in accordance with claim 8, wherein pitch vapours arising in said mixing and forming unit (8) are supplied to the output pipe (13) of said heat exchanger (11) through a pipeline (10).
 10. The device in accordance with claim 1, wherein an unprocessed proportion of anode waste is flowable into said mixing and forming unit (8) as well as the heated petroleum coke fractions (5, 6).
 11. A control system (2) for using waste heat of the annular anode furnace (3), in the device (1) according to claim 5, the control system comprising said feed pipe (12) through which the waste heat of the annular anode furnace (3) is drawn from the heating-up zone (22) and/or cooling-down zone (24), and a cleaning unit (14) through which the cooled and cleaned flue gas flows into a surrounding area, wherein at least one gas regulator (27, 28, 29, 30) is integrated in said feed pipe (12), and an opening angle of the gas regulator (27-30) can be set using a control system (2), a plurality of volume and/or temperature measuring sensors (19) being assigned to the control system (2) by means of which volumetric flow and the temperature of the flue gas can be measured in the individual sections of said feed pipe (12), the waste heat from the heating-up zone (22) and/or the hot air of the cooling-down zone (24) of the annular anode furnace (3) being channelled through said feed pipe (12) to said heat exchanger (11) arranged ahead of said flue gas cleaning unit (14), the petroleum coke (5, 6) required for forming the anode (4) being heated to a specified temperature value in said heat exchanger (11) by the effect of the flue gas flowing around the petroleum coke (5, 6) directly in said heat exchanger (11), the petroleum coke (5, 6) heated in this manner being formed into the anode (4) in said subsequent mixing and forming unit (8) and, depending on the measured volumetric flow and/or temperature of the formed anode (4), the control system (2) controlling the opening angles of the relevant gas regulators (27-30).
 12. The control system in accordance with claim 11, wherein the control system (2) is provided with a software and/or microprocessor by means of one or both of which utilisation of the waste heat from the annular anode furnace (3) can be adjusted according to measured volumetric flow and temperature of flue gas in corresponding sections of said feed pipe (12).
 13. The control system in accordance with claim 11, wherein a bypass pipe (21) is provided in said feed pipe (12) ahead of said heat exchanger (11), through which the waste heat is guided around said heat exchanger (11) directly into said flue gas cleaning unit (14), and volumetric flow in the bypass pipe (21) is adjusted by the control system (2).
 14. A process for using the waste heat from an annular anode furnace (3), the process comprising the steps of: channeling flue gas from a heating-up zone (22) and/or the waste heat from a cooling-down zone (24) of the annular anode furnace (3) into a heat exchanger (11); inputting petroleum coke fractions (5, 6) into the heat exchanger (11) for heating therein by the flue gas; inputting the heated petroleum coke (5, 6) into a mixing and forming unit (8) in which the petroleum coke (5, 6) is formed into an unfired anode (4).
 15. The process in accordance with claim 14, wherein the temperature of the unfired anode (4) is measured and the temperature of the flue gas in a feed pipe (12) is adjusted according to the measured temperature of the unfired anode (4).
 16. The process in accordance with claim 15, wherein the temperature of the flue gas in the feed pipe (12) is influenced by heating with a heater (20) or by admixing cold air from the surroundings. 